Excavata: 1st supergroup of protists

Estimated Reading Time: 2 Minutes

Welcome to our continuing AP Biology/ General Biology series here on STEM Talks! The purpose of this post is to discuss the supergroup of protists called Excavata. It is important to focus on the traits that distinguish the protists in the particular clades.

Excavates include protists with modified mitochondria and protists with unique flagella. Overall, support for the excavate supergroup is weak, making it one of the more controversial of the five clades. There are three major clades in the Excavata supergroup, which I will go into more detail below.

Diplomonads and Parabasalids

The protists in these two clades lack plastids (implying they are heterotrophic) and have modified mitochondria. Most diplomonads and parabasalids are anaerobic.

Diplomonads have modified mitochondria called mitosomes. Since these organelles lack functional ETC and hence cannot use oxygen in cellular respiration, diplomonads get the energy they need from anaerobic pathways. In addition, diplomonads have two equally sized nuclei and multiple flagella. Recall that a flagellum is an extension of the cytoplasm, consisting of bundles of microtubules covered by the cell’s plasma membrane. Many diplomonads are parasitic.

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A picture of a diplomonad

Parabasalids have reduced mitochondria called hydrogenosomes, which generate energy anaerobically and release hydrogen gas as a by-product. Just like diplomonads, most parabasalids are parasitic.

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A picture of a parabasalid

Euglenozoans

Protists that belong to this diverse clade includes predatory heterotrophs, photosynthetic autotrophs, and parasites. The thing that distinguishes euglenozoans is that they have a crystalline rod of unknown function inside their flagella. The two best-studied groups of euglenozoans are kinetoplastids and euglenids.

Kinetoplastids have a single, large mitochondrion that contains an organized mass of DNA called kinetoplast. These protists include species that feed on prokaryotes and also species that parasitize animals, plants and other protists. An example of a kinetoplastid is the Trypanosoma, which caused African Sleeping Sickness.

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Example of a kinetoplastid

Euglenids have a pocket at one end of the cell from which one of two flagella emerge. Euglenids tend to be mixotrophs.

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In the next post, I will discuss the Chromalveolata supergroup. Feel free to leave any question down below in the comments!

-Tahmid Islam

Co-founder and Editor of StemTalksNC

Credits:

Euglena picture: http://vsprotista.weebly.com/euglenids.html

Kinetoplastid picture: https://www.tulane.edu/~wiser/protozoology/notes/kinet.html

Diplomonad picture: https://www.78stepshealth.us/plasma-membrane/diplomonads-and-parabasalids.html

Parabasalid picture: https://microbewiki.kenyon.edu/index.php/Parabasalia

In all my posts, I follow multiple sources for AP Biology. However, I mainly go off of the Campbell Biology 8th and 11th edition.

 

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